Provides default implementations of ExecutorService
execution methods. This class implements the submit,
invokeAny and invokeAll methods using a
RunnableFuture returned by newTaskFor, which defaults
to the FutureTask class provided in this package. For example,
the implementation of submit(Runnable) creates an
associated RunnableFuture that is executed and
returned. Subclasses may override the newTaskFor methods
to return RunnableFuture implementations other than
FutureTask.

Extension example. Here is a sketch of a class
that customizes ThreadPoolExecutor to use
a CustomTask class instead of the default FutureTask:

invokeAny

Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do. Upon normal or exceptional return,
tasks that have not completed are cancelled.
The results of this method are undefined if the given
collection is modified while this operation is in progress.

Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do before the given timeout elapses.
Upon normal or exceptional return, tasks that have not
completed are cancelled.
The results of this method are undefined if the given
collection is modified while this operation is in progress.

invokeAll

Executes the given tasks, returning a list of Futures holding
their status and results when all complete.
Future.isDone() is true for each
element of the returned list.
Note that a completed task could have
terminated either normally or by throwing an exception.
The results of this method are undefined if the given
collection is modified while this operation is in progress.

invokeAll

Executes the given tasks, returning a list of Futures holding
their status and results
when all complete or the timeout expires, whichever happens first.
Future.isDone() is true for each
element of the returned list.
Upon return, tasks that have not completed are cancelled.
Note that a completed task could have
terminated either normally or by throwing an exception.
The results of this method are undefined if the given
collection is modified while this operation is in progress.

a list of Futures representing the tasks, in the same
sequential order as produced by the iterator for the
given task list. If the operation did not time out,
each task will have completed. If it did time out, some
of these tasks will not have completed.